The present invention relates to a calibrated sliding bearing bushing and a calibrating tool for producing the sliding bearing bushing.
The patent document WO 98/40751 discloses a small electric small motor which has an armature, a shaft supports the armature, and a first and a second sliding bearing bushing which rotatably support the shaft. The first sliding bearing bushing which is arranged to take a greater bearing load is outwardly formed as a calotte and arranged pivotably in the small motor to compensate for a canted orientation of the shaft relative to a construction axis of the small electric motor by self orientation of the first sliding bearing bushing. It is known that with this compensation a favorable load distribution within the sliding bearing bushing is provided. The second sliding bearing bushing is formed cylindrically on the outside and rigidly mounted in the small motor. The second bearing bushing is inwardly, over a part of its length, cylindrical for supporting the shaft. As seen from the first sliding bearing bushing, it is formed on another part of its length as a hollow cone, so that an inlet funnel is provided for easy insertion of the shaft during the assembly of the small electric motor. Because of the rigid connection of the second sliding bearing bushing, it is not possible to provide the self orientation corresponding to the self orientation of the first sliding bearing bushing. At the same time, a disadvantageously high surface pressure is provided in the cylindrical sliding bearing bushing depending on the accidental canted orientation of the shaft relative to the cylindrical sliding bearing bushing surface. It can result in wear of the sliding bearing material or the shaft material. This can lead to a non-quite running of the shaft and to dirtying a bearing gap which is unavoidably formed between the sliding bearing bushing and the shaft, and can ultimatelylead to bearing damages.
U.S. Pat. No. 4,293,789 discloses a small electric motor with an outer cylindrical sliding bearing bushing, which is formed as a hollow cone at both end sides and has between them a narrow longitudinal portion with a hollow cylindrical shape to provide a sliding bearing surface. FIG. 2 of this patent illustrates ring edges which limit the inner cylindrical longitudinal portion from both hollow-conical longitudinal portions. An unavoidable canted position of the armature shaft relative to the sliding bearing bushing can lead to the above mentioned disadvantage.
Also, experts in the art are familiar with sintered sliding bearing bushings which are impregnated with lubricant for calibration of the diameter of the sliding bearing surfaces by pressing of balls or calibrating mandrels with cylindrical cross-sections, to produce ring edges. The calibrating mandrels or balls compact the sintered sliding bearing material. Ring edges are produced, as shown for example in FIG. 2 of this patent.
European patent document EP 0 509 683 82 and in particular in FIG. 6 discloses a small electric motor with an outer cylindrical sliding bearing bushing, which has an inner curved surface as a bearing bushing surface, so that the sliding bearing bushing can be produced from a torus with a circular cross section by removing of a material of an outer periphery and in the end side regions. Because of the curved sliding bearing surface, no ring edges are available in the immediate vicinity to a shaft to be supported and no edge pressure of the above described type is possible. A disadvantage of this solution is that, due to the curvature of the sliding bearing surface, a sliding surface pressure is basically greater than that during the use of a canting-movable bearing bushings with cylindrical sliding bearing surfaces.
Accordingly, it is an object of the present invention to provide a sliding bearing bushing and a method of producing the same which eliminates the disadvantages of the prior art.
In keeping with these objects and with others which will become apparent hereinafter, one feature of the present invention resides, briefly stated, in a sintered and calibrated bearing bushing which has a first end side; a second end side; an inwardly cylindrical second longitudinal portion which is calibrated by a material compression and adjoins said second end side; a substantially hollow conical first longitudinal portion which adjoins said first end side; and a third longitudinal portion which is arranged between said first and second longitudinal portions, said third longitudinal portion transferring bend-free from a contour of said second longitudinal portion into a continuous curvature so that, in a contour of said cylindrical portion straight lines located parallel to a longitudinal axis of the sliding bearing bushing are tangents to a curvature, and at least in a region of the tangents at the curvature the material of the sliding bearing bushing is compressed by calibration substantially identically strong as in said cylindrical longitudinal portion.
When the sliding bearing bushing is designed in accordance with the present invention, it has the advantage that on the one hand when practically no canted position of the shaft for the sliding bearing bushing is available it has a favorable distribution of pressures, and on the other hand when not too great canting position of the shaft relative to the sliding bearing bushing is provided it has an operational property comparable with the operational property of the sliding bearing bushing disclosed in the European patent document EP 0 509 683. It can be indicated that the inventive sliding bearing bushing combines the advantageous properties of an inwardly cylindrical sliding bearing bushing with the advantages of a sliding bearing bushing provided with a continuously curved sliding bearing surface, and a corresponding orientation of the shaft relative to the sliding bearing bushing determines which of the both positive properties is used.
In accordance with another feature of present invention a sintered blank from which the sliding bearing bushing is formed by calibration, is formed substantially over its whole length substantially cylindrically inside. When the sliding bearing bushing is designed in accordance with this feature, then it has the advantage that the sintered sliding bearing material in the region of the continuous curvature and adjoining to the inner cylindrical longitudinal portion has a material compression which is at least as high as the material compression in the region of the cylindrical longitudinal portion and therefore an advantageous resistance to wear is provided in the region of the continuous curvature.
In accordance with the present invention also a calibrating mandrel is provided which has a cylindrical longitudinal portion; a continuously curved transition which is bend-free adjoins said cylindrical longitudinal portion and has a greater diameter than said cylindrical longitudinal portion, so that for a bend-free construction a projection of said cylindrical longitudinal portion in a projection direction encloses tangents to said continuously curved transition.
The calibration mandrel is designed for an edgeless transition from the cylindrical longitudinal portion to the longitudinal portion with the curved transition. This calibrating mandrel can be produced for example mathematically accurately with the use of a grinding roller oriented substantially transversely to the calibrating mandrel. It produces the diameter of the cylindrical longitudinal portion and determines the half diameter of the continuous curvature of the transition.
In accordance with a further feature of the present invention the calibrating mandrel has a substantially conical longitudinal portion having an increasing diameter and adjoining said continuously curved longitudinal portion. Such a calibrating mandrel has an advantage that a hollow-conical inlet funnel of the sliding bearing bushing is also compressed and smooth, and counteracts an undesirable escape of lubricant from a surface of the sliding bearing bushing which is not in contact with the shaft.
The novel features which are considered as characteristic for the present invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.